1. Field of the Invention
This invention relates to an alignment system for aligning a substrate to be exposed in an exposure apparatus used for the manufacture of semiconductor elements, liquid crystal elements, etc., particularly an exposure apparatus of step-and-repeat or step-and-scan type.
2. Related Background Art
In an exposure apparatus of this type, circuit patterns called masks or reticles, drawn on a master plate, are fixed on a photoresist layer of a semiconductor wafer and are developed to obtain desired photoresist patterns.
In general manufacture of semiconductor elements, several to several ten layers of circuit patterns are laminated. Therefore, it is necessary to accurately align an optical image of a circuit pattern, to which exposure is to be made, to circuit patterns already formed on a wafer. Various devices that are necessary for this alignment or matching are called positioning or alignment system. The alignment system is essential for an exposure apparatus, in which exposure under alignment as noted above is performed, and recently it is improved to be capable of highly accurate and high speed processing. The alignment system may roughly consist of three elementary techniques. One of these techniques is an optical system for alignment, which optically detects alignment marks formed on a wafer in advance and produces a photoelectric signal corresponding to an arrangement pattern of the marks. Another technique is a signal processing system, which electrically processes the photoelectric signal with an adequate algorithm to obtain a deviation from the regular position of the alignment marks. The last technique is an aligning mechanism for accurately correcting the position of the water or positions of masks or reticles according to the determined deviation.
Recently, extensive use has become made in plants dealing with semiconductor of a contracting exposure system (or also called stepper), which is an organic and high technical level combination of the above three elementary techniques, i.e., optical system for alignment as optical technique, signal processing system as electronic technique (or data processing technique) and positioning mechanism as precision machine technique. In the contracting exposure system, a light image of a circuit pattern of reticles is focused on an area (called shot area) of wafer through a high resolution projecting lens (with an aperture number of 0.35 to 0.5). The exposure shot of each cycle is or the order of 15 mm by 15 mm, and a stage with a wafer set thereon is stepped two-dimensionally in x- and y-axis directions to exposure the entire wafer surface. As practical form of optical system for alignment incorporated in the contracting exposure system, roughly there are three different systems. A first one of these optical systems is a TTR (through the reticle) system, in which alignment marks formed on a reticle and those on a wafer are simultaneously observed or detected through a projecting lens. A second optical system is a TTL (through the lens) system, in which no alignment mark on reticle is detected but sole alignment marks on wafer are detected. A third optical system is an off-axis system, in which sole alignment marks on wafer are detected through a microscopic objective lens provided separately and at a constant distance from a projecting lens.
In the TTR and TTL systems noted above, in which the wafer marks are detected through a projecting lens, light for illuminating the wafer marks is limited to a coherent laser beam (mono-wavelength) or g- or i-ray spectrum (psuedo-monochromatic) of a mercury lamp used for exposure. This is because the projecting lens is designed that its chromatic aberration with respect to exposure light (g- or i-rays) is best. Examples of the TTR system are disclosed in U.S. Pat. Nos. 4,566,795 and 4,402,596, and examples of the TTL system are disclosed in U.S. Pat. Nos. 4,677,301, 4,780,617 and 4,655,598.
Generally, with the TTR and TTL systems, which have resort to a projecting lens for wafer mark detection, marks belonging to a given shot area in a shot area array on a wafer can be detected comparatively freely by moving the wafer stage. In contrast, the off-axis system, in which marks (for instance two or three marks) belonging to only a predetermined shot area on wafer are detected usually due to a problem of a stroke of wafer stage movement. More specifically, it is possible, when the stroke of wafer stage movement is increased in this system, to permit detection of marks in a given shot area on wafer freely while permitting effective use of the advantage of the system in view of the illumination light of the optical system for alignment. However, a usual contracting exposure system uses a laser beam interference type instrument (called interferometer) for the measurement of the coordinate positions of the wafer stage. Therefore to increase the stroke of wafer stage movement dictates increasing size of a movable mirror (i.e., optical ) secured to wafer stage and hence the size of a wafer stage support base. Nevertheless, the advantage of the off-axis system, i.e., improvement of the mark detection accuracy obtainable with a free illumination arrangement, can not be ignored.
As shown above, the off-axis system can also detect marks belonging to a given shot area on wafer in case where the stroke of wafer stage movement is increased. In this case, however, a long time is required for mark detection depending on the arrangement and way of illumination of the optical system for alignment. In the off-axis system, the optical system for alignment is spaced apart from a pattern projection area of a photographing lens. This means that a corresponding extra distance has to be covered by the wafer stage. Therefore, a throughput reduction due to overall arrangement of the system is essentially inevitable. Considerations, therefore, are paid for ways, in which to reduce the mark detection time. The shortest mark detection time can be obtained by a system, in which marks on wafer are scanned by a slit-like converged spot of a laser beam for photoelectrically detecting scattered and diffracted light from mark edges as according to the prior-art references (1) to (5) and (7). This system is capable of mark detection with a practically sufficient signal-to-noise ratio owing to high brightness property of the laser beam. However, there is a significant problem in that a laser beam is usually a single wavelength beam. More specifically, thin film interference (multiple interference) or the like is produced due to a photoresist layers 1 to 5 microns thick covering the entire wafer surface. This phenomenon causes unexpected distortion of the light intensity distribution of the scattered and diffracted light from the mark edges (and hence unexpected waveform distortion of a corresponding photoelectric signal). The distribution of the light intensity distribution may not occur with any wafer and varies depending on the kind and thickness of the ground layer and photoresist layer of wafer. Further, usually distortion, if any, occurs in various ways. Therefore, no advantage over the prior art references (1) to (5) and (7) can be obtained by merely adopting a single-wavelength laser beam as noted above or psuedo monochromatic light (such as the wavelength spectrum of a mercury lamp) as mark illumination light (either spot light or uniform illumination light) of the optical system for alignment in the off-axis system. Rather, doing so merely leads to such disadvantage as a throughput reduction due to an extra stroke of wafer stage movement.
Accordingly, it is considered to have resort to light having a broad wavelength distribution in a bandwidth of about 200 nm as mark illumination light of the off-axis system. Doing so permits great reduction of adverse effects of the photoresist layer and improves the mark detection accuracy. That is, considerably high accuracy alignment can be expected by permitting detection of marks belonging to a plurality of given shot areas on wafer with wide band illumination light of the aligning optical system of the off-axis system.
However, as noted before it is not the case that distortion of the distribution of intensity of light from mark edges occurs with any wafer even in case of using single waveform or psuedo monochromatic light. In addition, even if there is slight distortion, its adverse effects can be alleviated with an adequate mark position detection algorithm. From these considerations, it is very illogical to substitute the aligning optical system of the off-axis system using wide band light for the whole mark detection for doing so inevitably leads to a throughput reduction even with wafers free from occurrence of distortion or wafers less subject to adverse effects of distortion.